As shown in FIG. 10, conventional ladder-type elastic wave filter 100 includes piezoelectric substrate 101, series elastic-wave resonator 102a formed on piezoelectric substrate 101, dielectric film 103a formed on piezoelectric substrate 101 so as to cover series elastic-wave resonator 102a, parallel elastic-wave resonator 102b formed on piezoelectric substrate 101, and dielectric film 103b formed on piezoelectric substrate 101 so as to cover parallel elastic-wave resonator 102b. 
In this configuration, film thickness Ha of dielectric film 103a is formed so that the frequency-temperature coefficient of series elastic-wave resonator 102a becomes 0. Film thickness Hb of dielectric film 103b is formed thinner than film thickness Ha so that the frequency-temperature coefficient of parallel elastic-wave resonator 102b becomes 0.
Here, there is known patent literature 1 for example as information on prior art documents related to the invention.
In a conventional ladder-type elastic wave filter, its ohmic loss increases at high temperature, which causes its passing characteristic to deteriorate.